Plasma reactors for processing silicon wafers in the manufacture of microelectronic integrated circuits typically employ materials or process gases such as fluorine or chlorine at some point during their operation. Likewise, materials or process gases such as fluorine or chlorine are used to periodically clean reactor chamber surfaces in non-etch applications as well. For example, a plasma reactor for etching either silicon or silicon dioxide thin films typically employs a fluorine-containing process gas during etch processing. As another example, a plasma reactor for chemical vapor deposition employs fluorine-containing gases during chamber cleaning operations. The problem is that fluorine is a highly efficient etch species and tends to etch not only the intended material, e.g., a thin film on the wafer or contamination on chamber walls, but also the chamber walls themselves, as well as other components of the chamber, such as replaceable process kits, shadow rings and so forth. Chamber walls and the various permanent and semi-permanent or replaceable components within the chamber are typically formed of materials that have certain desirable characteristics, such as their ability to act as an electrical insulator while withstanding heat and corrosion from the plasma. Such insulative chamber component materials must be strong and have a fairly high fracture toughness so that they can be machined without cracking, chipping, or other damage. For example, insulative chamber walls and chamber components used in etch processes are typically formed of dielectric materials or ceramics such as alumina, aluminum nitride, silicon nitride and the like. These materials have a fracture toughness of at least about 2 megaPascals-m1/2 or more, and therefore readily satisfy the requirement of machinability. Fracture toughness of a material is the measure of force required to lengthen a crack through the material by a given distance.
Since the materials used for internal chamber components are susceptible to corrosion and wear by fluorine-containing process gases and plasmas, the components must be periodically replaced, involving significant costs in manufacture as well as loss of use of the reactor during parts removal and replacement. Therefore, the industry has sought for a solution that would reduce or eliminate the need to replace such parts. To this end, various attempts have been made to find suitable protective coatings to deposit on the surfaces of chamber walls and chamber components to protect them from erosion, corrosion, wear and deterioration. Such protective coatings must be of materials that are less susceptible to attack by materials such as fluorine or chlorine, for example. The chamber components could be machined prior to deposition of the protective coating, so that the coating material itself would not need to be machinable, and therefore could have a relatively low fracture toughness. The main requirement for the coating, therefore, would be only that it resist or be impervious to corrosion by fluorine-containing gases. In fact, coating materials that have been tried thus far, such as, for example, yttrium oxide and yttrium aluminum garnet, have good resistance to fluorine while having poor fracture toughness. However, the low fracture toughness is not particularly relevant provided the coating is applied to parts that have already been machined.
Because of the poor fracture toughness of such fluorine-resistant materials, it has not been practical to form monolithic chamber components (e.g., chamber walls, process kits, focus rings, etc.) from such ceramic materials in their pure composition. For example, attempts to form a typical process kit dielectric ring from pure solid yttria fail because the yttria fractures during machining or milling. This difficulty occurs for a wide variety of yttrium aluminates, and is due in part to the nature of the yttria and alumina powders used to form such compounds. To form a yttrium aluminate ceramic, yttria and alumina powders are mixed in a desired proportion with a binder, such as water or alcohol, and pressed together in a mold to form the desired shape, the resulting component being referred to in the industry as a xe2x80x9cgreenxe2x80x9d part. The green part is then subjected to high temperature and at the same time, in some instances, high chamber pressure. This last step results in the green part becoming a hard ceramic part. The poor fracture toughness encountered with such ceramic parts is due in part to a low packing density of the green part formed by pressing the alumina and yttria powders. The hard ceramic is then machined, such as by cutting, grinding, drilling, milling, etc., to finished dimensions.
Ceramics made from mixtures of yttria and alumina powders form different ceramic compounds for different proportions of yttria and alumina in the mix. For example, a mixture of about 50% yttria and 50% alumina by mole percentage results, after sintering, in formation of ceramic yttrium aluminum perovskite. A mixture of about 38% yttria and 62% alumina results in ceramic yttrium aluminum garnet. Of course, pressing and sintering pure yttria powder results in ceramic yttria. We have confirmed the low fracture toughness of each of the foregoing ceramic compounds. Specifically, ceramic yttrium aluminum garnet has a fracture toughness of 1.7 megaPascal-m1/2 before heating treating and 2.0 megaPascal-m1/2 after heat treating, both values of which are insufficient for machinability. Ceramic yttria has a fracture toughness of 1.7 megaPasacal-m1/2 after heat treating. Ceramic yttrium aluminum perovskite is so weak that we would not measure its fracture toughness. We feel that a fracture toughness of at least 2.5 megaPascal-m1/2 is necessary for machinability. Thus, none of the foregoing compounds are machinable. These compounds span a wide range of mixtures of yttria and alumina powders, and therefore it would seem unlikely that some mixture thereof exists that could yield a sufficient fracture toughness for machinability.
One solution to the poor fracture toughness of such materials is to introduce another material, such as a silicon-containing species, into the yttria-alumina powder mix prior to pressure molding and heat treatment. The resulting ceramic material in some cases has a sufficiently high fracture toughness (e.g., 2 megaPascal-m1/2 to be machinable. Unfortunately, the added material is not as immune to attack by fluorine gas as is the basic yttrium-alumina compound. Accordingly, a chamber component monolithically formed of such a ceramic material (i.e., a yttrium aluminate ceramic containing an additive such as silicon), when introduced into a plasma reactor environment such as a fluorine-rich environment or a chlorine-rich, for example, is attacked by the fluorine or chlorine, and is thus hardly an improvement over conventional ceramic materials (e.g., silicon-nitride and aluminum nitride) currently used for chamber components and walls.
As a result, it has seemed impossible to produce a monolithic yttrium aluminate ceramic chamber component, such as a process kit or a ring, that can be machined to the required shape and size while being corrosion resistant in a fluorine-rich or chlorine-rich or other corrosive species-rich environment.
A component of a plasma reactor chamber for processing a semiconductor workpiece, the component being a monolithic ceramic piece formed from a mixture of yttrium aluminum perovskite (YAP) and yttrium aluminum garnet (YAG) formed from a mixture of yttria and alumina powders, the ratio the powders in said mixture being within a range between one ratio at which at least nearly pure yttrium aluminum perovskite is formed and another ratio at which at least nearly pure yttrium aluminum garnet is formed.
In accordance with one aspect, a monolithic ceramic component of a plasma reactor chamber for processing a semiconductor workpiece is produced by a process of forming a mixture of a yttria powder and an alumina powder in a ratio of yttria to alumina powders lying within a range between 50%-50% and 62%-48% mole percentage, pressing the mixture together in a mold to form a green body thereof and then heat treating the green body to form a hardened ceramic comprising a mixture of yttrium aluminum perovskite (YAP) and yttrium aluminum garnet (YAG). The mixture of yttria and alumina powders can be about 57% yttria and 47% alumina by mole percentage. The ratio of yttria and alumina powders in said mixture can be such that the hardened ceramic has a fracture toughness sufficient to be machinable. The fracture toughness can exceed 2.0 megaPascal-m1/ and can be about 3.6 megaPascal-m1/2. The mixture of the powders can be such that after the heat treating, the hardened ceramic is a ratio of YAP to YAG material such that the monolithic ceramic piece has a fracture toughness of at least 2 megaPascal-m1/2. In one regime, the ratio of YAP to YAG is in a range between about 65%-35% and 55%-45% of YAP to YAG, respectively. In another regime, the ratio of YAP to YAG is in a range between about 70%-30% and 50%-50% of YAP to YAG, respectively. Such ratios are generally in mole percentages. For example, the ratio of YAP to YAG is approximately 60%-40% of YAP to YAG in mole percentages. The ratio can be such that the fracture toughness exceeds about 2.5 megaPascal-m1/2. Or, the ratio can be such that the fracture toughness exceeds about 3.0 megaPascal-m1/2. Specifically, the ratio is such that the fracture toughness is about 3.6 megaPascal-m1/2.